of Minerals in Igneous Mocks. 297 



These facts prove that in the presence of water anhydrous silica 

 is rendered fluid at a temperature so low as 300° C. Whether or not 

 under these circumstances water enters into its constitution is not 

 known, but after consolidation it consists of truly anhydrous silica 

 (Si Oo), though in a state of glass. 



That water is associated with the quartz of granite and other 

 plutonic rocks is well known, for this mineral is the chief home 

 of fluid cavities, which recall those described in the glassy silica 

 of Mr. Hunter's experiment, though in the quartz of igneous rocks 

 they are relatively far less abundant. The old explanation of the 

 late consolidation of quartz, which rested on the suggestion that 

 solution played as important a part as fusion in contributing to the 

 fluidity of igneous magmas, is to a certain extent thus supported by 

 experimental evidence. The mistake of the older school of petrologists 

 lay in attributing too subordinate a role to fusion, which we now 

 know plays the chief part ; but in making this nearer approach to 

 the truth there seems to me a possibility that we may have fallen 

 into the other extreme of neglecting to take into consideration the 

 influence of water, which may have played an important though 

 minor part. If this can be shown to be the case we shall be provided 

 with an explanation of anomalies, which hitherto have resisted all 

 analysis. To return to the case of ajleolite syenite, examples are 

 known of this rock in which the several minerals have crystallized 

 out in the order of their fusion-points ; on the other hand, examples 

 can as certainly be cited in which this order has been reversed, so 

 that sodalite has followed a^leolite ; aeleolite, orthoclase ; and 

 orthoclase, plagioclase felspar. 



No satisfactory explanation has yet been offered of this and 

 numerous similar cases, but if the order which follows that of the 

 fusion-points be looked upon as the normal, our task will be 

 simplified, since it will be reduced to finding special explanations 

 of particular cases. It is possible that these explanations will not 

 always be of the same nature, and I do not propose now to enter 

 into a detailed study of this question, but I may perhaps be allowed 

 to point out that several hypotheses would appear to be excluded. 

 The influence of the chemical constitution of the magma is sometimes 

 alluded to in general terms, though rarely analyzed in detail. That 

 it must be very limited in its application is suggested by the 

 experiment of Fouque and Levy, in which they showed that in 

 the case of two fused mixtures, one consisting of equal parts of 

 anorthite and augite and the other with twice as much anorthite as 

 augite, the order of consolidation was the same for both, the 

 anorthite, as might have been expected from its higher fusion- 

 point, crystallizing out in both cases before the augite. Change 

 of pressure is sometimes invoked, and considering the very large 

 reduction in volume which accompanies the consolidation of fused 

 silicates this might naturally be supposed to have some effect. If, 

 however, such were the case some law should be found distinguishing 

 the order of consolidation of minerals in a plutonic magma from that 

 in a volcanic flow : but hitherto no constant difference has been 



